Geometry and scanning behaviour in hooded crow(Corvus corone cornix)flocks

Abstract

Spatial dispersion, reciprocal orientations of individuals and «scanning» behaviour were analyzed in flock-feeding hooded crows under experimental field conditions. The study was carried out near Lodi (northern Italy). Hooded crows were attracted to the area by baiting with maize seeds. To assess exactly the positions and orientations of the crows in the flocks, a new photographic method was employed which obtains planar maps of the flocks from prospective images (slides, movies). The time individuals in the flocks devoted to vigilance for predators («scanning» behaviour) and the time elapsing between successive scanning events were also recorded for a sample of individuals in flocks of different sizes. Crows had an aggregated dispersion within the experimental area but tended to disperse uniformly within the area effectively occupied by the flock. A new method to quantify the reciprocal orientation is also proposed. Neighbouring crows avoided face to face and face away reciprocal orientations and maintained intermediate reciprocal orientations. The time allocated to «scanning» behaviour was negatively related and the duration of the inter-scan interval was positively related to the size of the flock. Both the geometry of flocks and the scanning behaviour of individuals were strongly influenced by social factors.